In underwater photography the requirements for the physical structure of the camera housings are substantially different for that of cameras not used under the water. Specifically, there is a requirement for the camera to be watertight and light transparent in the field of view of the camera. There are means known which enclose an otherwise non-underwater type camera for underwater use. It is typical of the known underwater camera enclosures to provide a transparent window through which the camera will photograph the desired underwater subject, as well as multiple external mechanical actuators to manipulate other controls of the camera enclosed therein. Furthermore, transparent openings in the underwater enclosures have been provided to allow self-contained flash units positioned upon the structure of the camera to illuminate the surrounding underwater area through the same enclosure containing the camera. Of particular interest are the enclosures for the recently developed self-developing cameras, wherein the film ejects from the camera upon completion of the exposure and commences developing by itself. An example of enclosures permitting underwater use of self-developing cameras are shown in U.S. Pat. Nos. 3,832,720 and 3,832,725, for use with the Polaroid Model "SX-70" self-developing instant camera. These patents are also applicable to the subsequently developed Polaroid Model "Pronto" and "660." Further advances in self-developing cameras include automatic focusing according to signals provided by an air-sonar range-finding device contained within the self-developing camera housing. These air-sonar focused cameras cannot be successfully used within the camera housings disclosed in the above-cited patents due to the great differences in acoustic propagation parameters of the media between the air-sonar transducer and the subject to be photographed. Specifically, the air-sonar signal must pass through a short distance of air, a plexiglass or other rigid and clear enclosure material, the water through a distance to the subject, and back in a reverse sequence to the air-sonar element. The typical result is for the air-sonar signal to be substantially entirely reflected within the camera's waterproof housing so as to cause the focusing mechanism of the self-developing cameras to focus at either the inner surface of the waterproofing housing or the maximal distance limit of the camera providing an improperly focused picture.
Adaptation of the underwater camera enclosure to include an acoustic window, such as described in U.S. Pat. No. 2,960,175, to permit the air-sonar transducer to communicate to the water media, will be unsuccessful due to the great difference in sound propagation velocity between the air and the water medium. The result of this difference in propagation velocity, assuming the acoustic window permits sufficient energy to traverse the boundary of the media, would be to cause the camera to focus much closer than desired for a particular underwater subject distance.
Underwater camera housings and enclosures incorporating acoustic or sonar range-finding devices are shown in U.S. Pat. Nos. 3,738,248 and 4,153,357. The enclosures disclosed therein provide a shutter signal to trigger a camera having a set predetermined focal distance. In operation, the apparatus requires the camera to be maneuvered into position with a distance to the underwater subject equal to predetermined focal length, at which time the camera is triggered and the photograph is taken. The apparatus disclosed in the above-cited patents provide no means to communicate a measured distance through the water to the subject, nor means for providing a signal proportional to that distance to the enclosed camera in a manner to adjust or affect the focusing of that enclosed camera.
Additionally, there are differences in the optical properties of the air and the water medium by which the focus of the cameras calibrated in the air media must be adjusted or offset to provide correct focusing through the water media.